Borrowing from Latin littera canīna meaning "letter pertaining to a dog", "a dog's letter" or "a canine letter".
dog's letter (uncountable)
- (rare) the letter R, which was trilled in Latin, imitating the sound of a snarling dog.
1640, Samuel Ramsey, The English Language and English Grammar, New York and London: G.P. Putnam's Sons, translation of original by Ben Jonson, published 1892, page 168:
- R is the Dog's letter, and hurreth in the sound the tongue striking the inner palate, with a trembling about the teeth.
1858, Rosina Bulwer Lytton, The World and His Wife, Or, A Person of Consequence […] , volume 2, London: Charles J. Skeet, page 19:
- And in this short query the dog's letter, the r, whirred through the air, like the ricket of a pheasant rising; […]